hotpoint mistral fridge freezer?

My hotpoint mistral fridge freezer will freeze food to the point it is covered with ice/snow and the fridge is hot +20 degrees....The fridge freezer was defrosted and had a new thermostat replaced by a hotpoint engineer which cost me £70 but the problems has returned again and again. I have reset the appliance by turning the appliance off then holding in the zone and save buttons and turning the appliance on again holding buttons for 10 sec. Is it best I cut my loses and buy another appliance or is there a fix for this problem. It is only 2yrs old